The present invention relates to a focus servomechanism control system for controlling a focus of light beams impinging on a recording film of an optical disc in an optical disc system. More particularly, this invention relates to a focus servomechanism control system having an automatic offset setting means, for automatically determining an optimum offset value, which generates an optimum focus of light beams impinging on the recording film, and setting the determined optimum offset value to a light beam focusing means.
An optical disc system used as a large-capacity external memory device in an information processing system can record at a density more than ten times that of the recording density attainable by a conventional magnetic disc system.
Data recording in the optical disc system is accomplished, for example, by applying strong light beams, i.e., at a write power level, from a laser diode to tracks defined in a recording film on an optical disc to form holes or pits in the tracks in the recording film by thermal reaction. The data thus written in the tracks in the recording film can be read from reflected beams when weak laser beams emitted from the laser diode are applied as optical beams, having a read power level lower than the write power level, to the tracks.
The tracks are defined with a small distance between adjacent tracks, for example, several micron meters, in a radial direction of the optical disc. A track servomechanism control system is provided to precisely control the positioning of an optical head emitting laser beams and receiving the reflected beams with a corresponding track.
In addition, a focus servomechanism control system is provided to precisely control the focusing of laser beams emitted from the optical head onto the tracks to ensure data recording on and data reproduction from the optical disc. In the focus servomechanism control system, offsetting becomes necessary due to, for example, the displacement of a light sensing element and inner circuits in the focus servomechanism. Accordingly, in the focus servomechanism control system, an offset compensation must be provided to improve the accuracy of the data recording and reproduction. When determining whether or not the focusing is at an optimum value, a focus servomechanism signal which is used for controlling the focus is not used because this signal may contain offsets. Instead, a tracking servomechanism signal, more specifically, a tracking error signal, is used for determining the focus. A preformat is recorded in each track, and a light measurement circuit receives the preformat signal, detects a deviation between a light beam axis and the track, and produces the tracking error signal. It is considered that the optimum focus is obtained when the tracking error signal is at a maximum amplitude.
In the prior art, the detection of the optimum focus is performed manually, for example, by changing a variable resistor for a focus offset adjustment, observing the tracking error signal on an oscilloscope, determining a value of the variable resistor which produces the maximum amplitude of the tracking error signal, and finally, locking the variable resistor by using an adherent such as paint.
The prior art offset adjustment suffers from the disadvantages of a low accuracy for the optimum offset value, a low operation efficiency, and a lack of flexibility for readjustment when the offset is varied after an initial offset adjustment has been performed.